Method of positioning electrode assembly in transmit-receive devices



y 1957 H. J. MCCARTHY 2,801,147

- METHOD OF POSITIONING ELECTRODE ASSEMBLY IN TRANSMIT-RECEIVE DEVICES Filed Aug. 2, 1955 FIG. I

U INVENTOR.

HENRY. J. M CARTHY ATTORNEY United States Patent METHOD OF POSITIONING ELECTRODE ASSEM- BLY IN TRANSNHT-RECEIVE DEVICES Henry J. McCarthy, Danvers, Mass., assignor to Bomac Laboratories Inc., Beverly, Mass., a corporation of Massachusetts Application August 2, 1955, Serial No. 526,051

1 Claim. (Cl. 316-1) The present invention relates to gaseous discharge switching devices and more particularly to a novel method of positioning electrode structure within such devices.

In radio direction and ranging systems, switching devices are employed to permit the use of a single antenna for transmitting and receiving high frequency radio signals. During transmission at the high power level, the sensitive receiver is decoupled by a device referred to in the art as a transmit-receive tube. Ionization of a gaseous medium within a resonant cavity effectively blocks the transmitted pulse from the receiver branch of the circuit, and permits essentially all of the signal power to pass to the antenna.

The transmit-receive tube commonly comprises a hermetically sealed envelope defining a resonant cavity with discharge gap electrode structure mounted therein and resonant dielectric window members positioned on opposed sides to permit passage of electromagnetic waves through the tube. During the transmission cycle, the high voltage gradient across the discharge gap reaches the ionization potential of a suitable gas filling with a resultant high intensity gaseous discharge.

Such devices further employ an auxiliary electrode positioned within one of the gap electrodes to supply a source of electrons in the vicinity of the discharge gap thereby reducing the time necessary for the gaseous breakdown. Generally a direct current voltage supply sufiicient to maintain a current of 100-200 microamperes is sufiicient for this ignitor action.

After the main radio frequency discharge returning signals at a power level below the threshold breakdown potential will be permitted to pass to the receiver. These signals are attenuated to a slight degree by the presence of ions in the discharge gap. In certain radar applications, however, it is desirable to attenuate such low level signals still further to prevent passage of energy. Since mechanical attenuating devices would be inoperative at time intervals in the order of microseconds, one solution has been to provide an ignitor electrode with a higher current density and to move this electrode in close proximity to the gap to thereby supply a much larger number of electrons. In a tube embodying such an ignitor-attenuator electrode approximately 7508S0 microamperes of direct current is provided and the inner tip is spaced a critically short distance from the discharge gap.

Accordingly, it is an object of the present invention to provide an improved method of fabricating transmitreceive tubes employing a combination ignitor-attenuator electrode.

A still further object is to provide a novel method of positioning an electrode in a transmit-receive tube which is accurate and efiicient without reliance on skilled assembly labor.

Another object is to provide a method of electrically adjusting the positioning of ignitor-attenuator electrodes in transmit-receive devices.

The invention attains the objects enumerated above by providing a method of positioning of the ignitorattenuator electrode by means of measuring the transmission of power through the tube resonant cavity prior to final assembly of the electrode. According to my invention, a local oscillator of low level power, such as a klystron is set at a resonant frequency with power output measured by means of a crystal detector. An electrode and reservoir assembly is positioned in its desired position at one end of the tube envelope with the electrode at a point where it will have little or no eifect on the transmitted power. The electrode assembly may then be moved in until the power is reduced from 10 to 15% of the original level. By returning the resonant cavity the trans mitted power should return to from to of the original value. If the power level is less than this value, the electrode has been inserted too far. Conversely if the power level returns to approximately the original value, the electrode has not been inserted far enough.

By thus determining the electrode positioning electrically greater accuracy may be attained than with former methods relying on operators judgment or crude mechanical arrangements.

The invention will now be described in detail by referring to the accompanying drawings in which:

Fig. 1 is a perspective view of the illustrative embodiment;

Fig. 2 is an enlarged exploded view partly in section of the illustrative embodiment;

Fig. 3 is a digrammatic presentation of the apparatus employed in the method of the invention.

Referring to the drawings Fig. 1 illustrates a transmitreceive device well-known in the art comprising a substantially cylindrical metal envelope 1 with a centrally located dielectric window 2 and a gas storage reservoir bulb 3. An ignitor electrode 4 supported by the reservoir bulb 3 extends axially inwardly and the invention deals with the critical positioning of this electrode within internal tube structure.

As shown in detail in Fig. 2, a hollow passageway 5 is provided in the tube envelope 1 with opposed wall members 6 and 7 having spaced convergent conical ends 8 and 9 positioned therein to define a resonant cavity 10. Wall member 7 may be formed of a deformable material to provide for tuning of the device by adjustment of the spacing between cones 8 and 9 to the resonant frequency by means of tuning mechanism within housing In the processing of the components of the embodiment, a reservoir subassembly is fabricated comprising a base member 12 having a cup section 13 which has sealed thereto by well-known glass-to-metal sealing techniques the reservoir bulb 3. lgnitor electrode 4 is sealed at its upper end to the bulb 3 with the other end 14 extending downwardly. The internal tube components may also be subassembled within the envelope. The final assembly operation comprises assembly of the reservoir assembly in the open end of passageway 5, positioning of the electrode end 14 within the envelope according to the teachings of the invention, permanently fixing the reservoir base 12 within passageway 5 as by brazing or soldering, exhausting the tube and filling same with an ionizable atmosphere by means of tubulation 15.

In the construction of a transmit-receive tube designed for operation solely as a switching device without the additional feature of low power level attenuation, the ignitor electrode tip is positioned a distance of approximately fifty (.050) thousandths of an inch from the apex of the cone electrode. This is necessary to avoid any so-called interaction of this electrode on low level re turning signals passing from the antenna to the receiver.

In certain radar applications, however, it is desirable to attenuate these weak returning signals by afactor of approximately 30 to 40 decibels. It has been observed that a high current passed through an ignitor electrode together with positioning closer to the. apex of the conical.

electrode will provide'theattenuation satisfactorily. However; as a result of the spacing of'the ignitor electrode near the discharge'gap another disadvantage exists in that high level radio frequencypowenmay. be coupled out of the cavity: similar to the effect of a coupling loop.

The positioning of the ignitor electrode is, therefore, extremely critical and'my invention provides an accurate method of accomplishing the desired result.

Fig. 3.illustrates the apparatus employed including a local oscillator 16', such as a klystron, coupled to a. fixed attenuator pad 17. The subassembled tube body 1' may be mounted between flanges 18. and 19 with the body rotated 90. fromv the plane shown. in Figs. 1 and 2. Another. fixed attenuator pad' 20 and a crystal detector 21 complete the radio frequency plumbing components. A measuring device, such as a microammeter 22, is

connected to crystal detector 21 by means of a coaxial then be obtained by setting themicroammeter at a given- .value of crystal current. The electrode is then gradually moved further into the cone passage thereby detuning the cavity'until the transmission is reduced by approximately 10 %-15% of the original level. The tube may then be retuned by means of the'screw mechanism 11 and the crystal currentreading should return. to approximately 90% to 95% of the original value. The reservoir assembly may be positioned at this point by tightening of. the screw 25 extending into passageway 5 and contacting base 12'. Y

If the value obtained with the electrode all the way in is greater than the %-95'% level the'criti'cal point has been exceeded and the electrode must be withdrawn. Conversely, a reading below the desired value indicates that the electrode insertion is not far enough to provide the necessary attenuation. in.thefcompleted tube; After determination of the correct setting by this electrical'procedure, the reservoir base 12 may be permanently, secured as by soldering or brazing the joiningedgeof base 12 and passageway 5 to provide a vacuum tight seal.

' Tubes fabricated according to the disclosed method of electrode positioning will produce very satisfactory re-. sults in the transmit-receive and attenuator What is claimed is":

A method of electrically controllingthe spacing of a combination ignitor-attenuator electrode adapted to attenuate radio frequency signals when a direct current of 750-850 microamperes is impressed thereon, within a transmit-receive tube. subassembly having spaced. internal discharge gap electrode structure disposed within a resonant cavity with at least one ofsaid electrodesprovided with an axial passageway, comprising the steps of'mounting said tube subassembly in a microwave transmission system including a tunable low level source of energy and a crystal detector, positioning the ignite]:-

attenuator electrode within said passageway, tuning said.

low level source to a reference fiequcncy, adjusting the gap electrode spacing to be. resonant-at this frequency,

and recording'the transmitted energy indicatedv by said crystal detector, inserting the i'gnitor attenuator electrode further into said passageway until the transmitted energy is reduced to. approximately 10% to 15% of the. original,

7 value, re-adjusting said tube assembly. gap electrode structure until approximately 9.0% to of the original value is attained and'securing said ignitor-attenuator electrode Within said-passageway when the desired setting has been obtained.

References Cited in the file of this patent UNITED ST ATES' PATENTS 2,617,038 Russell Nov.4,1'952' applications. 7 

